Because insulin is an important regulator of protein metabolism, we hypothesized that physiological modulation of insulin secretion, by means of extreme variations in dietary carbohydrate content, affects postabsorptive protein metabolism. Therefore, we studied the effects of three isocaloric diets with identical protein content and low-carbohydrate/high-fat (2% and 83% of total energy, respectively), intermediate-carbohydrate/intermediate-fat (44% and 41% of total energy, respectively), and high-carbohydrate/low-fat (85% and 0% of total energy, respectively) content in six healthy men. Whole body protein metabolism was assessed by 24-h urinary nitrogen excretion, postabsorptive leucine kinetics, and fibrinogen and albumin synthesis by infusion of [1-13C]leucine and [1-13C]valine.

The low-carbohydrate/high-fat diet resulted in lower absorptive and postabsorptive plasma insulin concentrations, and higher rates of nitrogen excretion compared with the other two diets: 15.3 ± 0.9 vs. 12.1 ± 1.1 (P = 0.03) and 10.8 ± 0.5 g/24 h (P = 0.005), respectively. Postabsorptive rates of appearance of leucine and of leucine oxidation were not different among the three diets. In addition, dietary carbohydrate content did not affect the synthesis rates of fibrinogen and albumin.

In conclusion, eucaloric carbohydrate deprivation increases 24-h nitrogen loss but does not affect postabsorptive protein metabolism at the hepatic and whole body level. By deduction, dietary carbohydrate is required for an optimal regulation of absorptive, rather than postabsorptive, protein metabolism.

-Very small sample size (n=6), though the error bars are remarkably small for having 2 subjects per condition

- They were limited to daily activity... i.e. no physical stimulus for growth. May or may not have an effect on protein absorption. This would definitely be interesting to play with.

Their results say that if you are not exercising and eat low carb, you don't absorb as much protein. Seems pretty intuitive. I don't think they prove that insulin is a necessary mediator of protein absorption, but repeating in diabetics/mouse model could do so.

I agree, the sample size was too small to draw any firm conclusions so it would be interesting to see further studies performed on a larger group of more active/athletic subjects. I doubt that the results would be all that different though because of the well documented protein sparing effect of carbohydrates.

Edit: I'm forgetting that energy balance plays an important role in maintaining nitrogen balance and that carbohdrates and fats have a simlar protein sparing effect in hypercaloric diets. Carbohydrates do however have a greater protein sparing effect than fats in eucaloric and hypocaloric diets so any further studies would need to take energy balance into account.

Who is going to try to cut off fat with just diet though? People that often rebound back up?

According to the findings of the US National Weight Control Registry the majority of people who were successful in losing 30lbs and maintaining that weight loss over a three year period reported that they followed a low-energy and low-fat diet (link) with only 10.8% of participants reporting losing weight after following a low-carbohydrate diet (link). Most of the participants in the study who regained weight reported an increase in the percentage of total calories coming from fat (link).

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Strength training is the most protein sparing thing when cutting weight....

Agreed.

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Macronutrient partition matters very little at that point, and higher fat diets are proven to drop more fat mass quicker.

While high-fat diets have been shown to result in greater weight loss than low-fat diets in the short term the differences between hypocaloric diets with different macronutrient intakes even out over time so that in the long term the extent and duration of the calorie deficit ultimately determines how much weight is lost.

I could be completely wrong, but I would imagine that exercise, though it breaks down our protein, likely stimulates some mechanism that would then enhance our bodies' ability to digest, absorb, and process protein. If there's a post workout window for carbs, I figure there's probably a post workout window for protein too, and maybe it lasts much longer than the carb window? Just a guess.

In any case, I know when I eat truly keto, my urine smells like ammonia, so I'm not surprised that not eating carbs increases nitrogen excretion. You're probably eating even more protein than you normally would as well.